5.4.3.5 Mid/MidB/Mid$/MidB$ Statement

Note

This section describes the implementation of MS-VBAL §5.4.3.5 Mid/MidB/Mid$/MidB$ Statement.

Syntax

AST node	Instruction kind(s)	Notes
MidStatementNode	Simple	Mid, Mid$, MidB and MidB$. See RD-VBAL §3.4.2 Simple Statements.

MidStatementNode records the statement's spelling in two independent flags: IsByteMode distinguishes MidB/MidB$ from Mid/Mid$, and IsStringInput records the $ suffix (Mid$/MidB$).

Spelling	IsByteMode	IsStringInput
Mid	false	false
Mid$	false	true
MidB	true	false
MidB$	true	true

The replacement-span mechanics of MS-VBAL §5.4.3.5 split only on byte mode (MidB/MidB$ versus Mid/Mid$), never on the $ suffix. IsStringInput is preserved because it mirrors the VBVariant/VBString split of the Mid/Mid$ function overloads, which matters for static semantics.

Static Semantics

The declared type of the target MUST be String or Variant (MS-VBAL §5.4.3.5); another is a TypeMismatch. The target is a variable: a literal, an operator's result or a constant is VBC09333 (VariableRequired).

The position and the length are used as the runtime semantics uses them - as Long - and the value as a String, so each has to be Let-coercible to that (RD-VBAL §5.5.1 Let-coercion).

The rule is MidStatementStaticSemantics's, which StatementStaticSemanticsEvaluator asks. An operand whose declared type is not known is deferred, not rejected.

Note

A Byte() target is a run-time possibility the specification's static rule does not allow: it is a TypeMismatch.

Runtime Semantics

Mid replaces a span of the characters of a variable with characters of a value. The replacement never changes the length of the string: the number of characters replaced, x, is the least of the length asked for, the number of characters left in the target from start on, and the number of characters in the value.

1. The target's value and the value to assign are each Let-coerced to String (RD-VBAL §5.4.3.8 Let Statement). A Variant is what it holds.
2. start and length are Let-coerced to Long.
3. If start is less than or equal to 0, or greater than the length of the target, or if length is less than 0, runtime error 5 (Invalid procedure call or argument) is raised. A target with no characters has no start that is in it.
4. The new string is Let-assigned to the target, so the target can be any variable expression a Let accepts, and a fixed-length String target is given back at its own width.

Spelling	Counts in
Mid, Mid$	Characters.
MidB, MidB$	Bytes of the string's in-memory form, two to a character, as LenB counts. A position can fall inside a character, which is split.

The $ suffix changes nothing at run time.

Implementation

Type or member	Role
RDCore.Runtime.Semantics.Statements.MidStatementRuntimeSemantics	The runtime semantics of all four spellings (RDCore.Runtime).
RDCore.Runtime.Semantics.Statements.StatementRuntimeSemanticsProvider	Dispatches a MidStatementNode; see RD-VBAL §3.5.4 Execution.

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⏮️ RD-VBAL §5.4.3.4 Erase Statement | ⏭️ RD-VBAL §5.4.3.6 LSet Statement